1. Field of the Invention
The present invention relates to a mark sensing device, a device for driving a turnable body and an image forming apparatus.
2. Description of the Prior Art
Today, an image forming apparatus of the type including a photoconductive belt, intermediate image transfer belt or similar turnable body for image formation is extensively used. A prerequisite with this type of image forming apparatus is that the amount of turn or movement of the turnable body be controlled accurately enough to precisely position an image on the turnable body or a recording medium being conveyed by the turnable body. In practice, however, the amount of turn of the turnable body often varies due to some cause and makes it difficult to reduce the shift of an image position. Particularly, in a color image forming apparatus, a change in the amount of rotation prevents images of different colors from being registered at a preselected position, i.e., causes the images of different colors to be shifted in position from each other.
Further, the moving speed, or amount of turn, of the photoconductive belt, intermediate image transfer belt or similar turnable body varies in accordance with, e.g., the variation of the thickness of the belt, the eccentricity of rollers or the irregular speed of a drive motor assigned to the turnable body. Particularly, in a color image forming apparatus, positioning errors ascribable to the irregular speed of the belt appear in the form of a waveform containing a plurality of frequency components. Images of different colors transferred to the belt whose speed is varying one above the other are not accurately registered, resulting in color shift, color variation or similar image defect.
In light of the above, Japanese Patent Laid-Open Publication No. 6-175427, for example, discloses an image forming apparatus in which a rotary encoder is directly connected to the shaft of a drive roller that drives a turnable body or similar rotary shaft. In this configuration, the angular velocity of the drive motor is controlled in accordance with the angular velocity of the turnable body sensed by the encoder. However, it is difficult with this prior art apparatus to accurately control the amount of turn or movement of the turnable body because it is only indirectly controlled via the control of the angular velocity of the drive motor.
To solve the problem stated above, Japanese Patent Laid-Open Publication Nos. 6-263281 and 9-114348 each teach a system configured to sense marks formed on the surface of a belt or turnable body with a sensor and calculate the surface velocity of the belt on the basis of the resulting pulse intervals for thereby feedback-controlling the amount of movement of the belt. This kind of system is capable of directly observing the behavior of the belt surface and therefore directly controlling the amount of turn or movement of the belt. However, neither one of the two Laid-Open Publications mentioned above teaches a method of forming the marks on the belt or a method of sensing the marks. Further, because a belt generally applied to, e.g., an image forming apparatus is flexible, deformable and irregular in thickness, the distance or the angle between the marks formed on the belt and the sensor is caused to vary.
Technologies relating to the present invention are also disclosed in, e.g., U.S. Pat. No. 3,107,259.